Light sources are frequently used in position-measuring units, whose light is modulated as a function of position by an appropriate measuring standard device, for example, in the form of a scale or a graduated disk. Usually, the modulated light is converted by photodetectors into electrical photocurrents, so that the relative position between the light source and the measuring standard device is able to be ascertained from these position-dependent photocurrents.
In such position-measuring units it may happen that the light intensity changes with time, for example, that it diminishes because of aging of the light source during the course of the operating time. Because of the changes of the light intensity received by the photodetectors, regulation of the light source is frequently undertaken to improve measuring quality.
U.K. Published Patent Application No. 2 054 135 describes a method and a corresponding device for regulating such a light source. For this purpose, the sum of the position-dependent photocurrents is formed in a control circuit as the actual value of the light intensity. This actual value is then compared to a non-changeable, fixedly predefined setpoint value, in order thereby to ascertain the system deviation for regulating the light source. Such a regulating device has the disadvantage that when it gets dirty, e.g., in the case of soiling over large areas of the measuring standard device, no position-measuring or only an inaccurate or unstable one is possible.